Most battery powered applications require reverse battery protection which protects equipment from damage due to a misconnected battery and the consequent polarity inversion. In many cases a simple diode in the +ve or −ve bus can offer adequate protection. In cases of large current systems, a FET may be used to reduce the voltage drop and thus reduce the power losses and consequently reduce the size of the heatsink.
FIG. 1 shows a typical configuration where such a such a scheme is used where the FET is in the −ve DC path which reduces the complexity of the drive circuit for the reverse battery FET.
Placing the reverse battery FET in the position of FIG. 1 (i.e. in the −ve rail) causes equal current in the reverse battery FET as in the main FET. This implies that a large amount of silicon needs to be used in conjunction with a larger heatsink.
In many systems the duration of the current flow through the freewheeling diode is of a low duty cycle (such as simple low frequency PWM fan controls, solid state relays (SSRs), engine start systems and load disconnect devices). In these application it is advantageous to use an alternate scheme.